Photovoltaic cells of the heterojunction type require the presence of two metal electrodes that make a low-resistance contact with the adjacent semiconductor material. If either metal electrode does not form a low-resistance contact, the characteristic I-V curve is altered and the cell does not function at its maximum potential efficiency. Of particular interest is the p-type CdTe/n-type CdS cell, because of the high conversion efficiencies that are now available as reported in my U.S. Pat. No. 4,207,119, issued June 10, 1980. One commercial drawback with such a cell has been that the preferred metal for making ohmic contact with the p-type CdTe layer has been gold, as it provides the lowest contact resistance for p-type CdTe when the preparation methods of my aforesaid patent are carried out. Other metals are useful, but they have been less desirable because their contact resistance is substantially greater. That is, a cell that uses a metal other than gold for the ohmic contact with p-type CdTe, can incur as much as a 50% loss in conversion efficiency. While such a cell is still functional, its low efficiency makes it uneconomical.
However, at least due to the rapid rise in the cost of gold, gold contacts are not economical. Therefore, there is a need to provide contacts between other less expensive metals and p-type CdTe, wherein contact with such metals have as low a resistance as the contact obtained when using gold as the electrode material.
Some study has been done concerning contacts for p-type CdTe layers. Although there was some success in contacting p-type CdTe in the single crystal form, low-resistance contact to polycrystalline, thin-film samples is recognized in Semiconductors & Semimetals, Zanio (1978), p. 203, as being much more difficult.
In the methods developed for making contacts to single-crystal CdTe samples, a chemical etching step often has been used. The etchants used, however, are unsuitable for thin-film or polycrystalline samples. They either etch away the material so rapidly that much of the thin-film sample is removed, constituting waste and a danger of destroying the entire film; or attack the grain boundary of the CdTe selectively, causing a high concentration of defects. Hot HNO.sub.3 is an example of such an etchant.